1. Field of the Invention
The present invention relates to a technique for inputting/outputting a high-speed differential signal, in particular, a differential transmission circuit capable of determining a cable length without an exclusive interface by using a general-purpose cable.
2. Description of the Related Art
In a high-speed differential signaling where a small potential difference, such as a few hundred millivolt, is sent and received as a signal through a cable, a sufficiently high driving performance is required to be set considering a signal attenuation caused by a transmission through a transmission cable. On the other hand, considering an effect of EMI (Electromagnetic interference), the driving performance is preferred to be as low as possible to reduce emission of an electromagnetic wave. Accordingly, “a preferably low setting value with which communication can be performed normally even if a signal is attenuated on a transmission cable” is to be selected for the driving performance to output a signal. However, an attenuation amount of a signal cannot be specified uniquely in the cable transmission when a length of the connected cable is unknown. In such a case, it becomes difficult to provide an appropriate setting value suited to the transmission.
To solve this problem, in the Japanese Patent Application Laid-open No. 2000-181591 (Patent Document 1), for example, a signal transmission cable includes another exclusive interface inside thereof for determining a cable length, and optimal outputting control is performed according to the cable length obtained by the interface.
A problem in the technique disclosed by the Patent Document 1 is that a special cable needs to be prepared because the exclusive interface has to be included in a signal cable, and a general-purpose cable with a low price cannot be used. If the general-purpose cable is used, another interface for measuring a cable length is required, and a system becomes enlarged.